Germanium crystal rectifiers and method of producing the crystal element thereof



Patented Mar. 4, 1952 GERMANIUM CRYSTAL RECTIFIERS AND METHOD vOF PRODUCING THE CRYSTAL ELEMENT THEREOF Douglas E. Jones and John W. Ryde, Wembley, England, assignors to Hazeltine Research, Inc., Chicago, 111., a corporation of Illinois No Drawing. Application July 29, 1947, Serial No. 764,580. In Great Britain July 16, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires July 16, 1961 Claims. 1

This invention relates to crystal rectifiers or detectors of the type suitable for use as rectifiers or mixers in circuits carrying electrical oscillations, for example in radio receivers. Such rectifiers or detectors comprise as one contact element a semiconducting crystalline mass associated in use with a contact element which is usually a metal point. The invention relates also to the manufacture of crystal rectifier elements of the type specified.

One crystalline material often used as a semiconductor of this type is silicon. Experiments with the closely allied element germanium have been reported, but the only statement comparing germanium with other materials that we have found is that of E. Merritt (Proceedings of the National Academy of Science, vol. II (1925) p. 743) Merritt states that the rectifying action of germanium is less marked than with many other materials, but the contacts are quite stable and the behavior at different points on the surface is more uniform than in the case of most crystal rectifiers.

Our experiments do not confirm these statements generally. In our experience germanium is no more stable or uniform than commercial silicon; on the other hand we have found that if the germanium is highly pure and preferably is subjected to a chemical etching treatment its rectifying action may be improved and is then often better than that of most of the commercial silicon we have examined.

Accordingly, it is an object of the present invention to provide a new and improved germanium crystal rectifier.

It is a further object of the invention to provide a germanium crystal rectifier having substantially improved sensitivity, or ratio of forward-to-backward conductance, and one characterized by the ability safely to conduct in its forward direction much higher values of current than heretofore readily attainable.

It is a further object of the invention to provide a new and improved method of producing crystal rectifier elements of the germanium type.

In accordance with the invention, a crystal rectifier element comprises a body of germanium having unremoved impurities in an amount not exceeding 0.10 per cent and having an etched crystallographic cleavage contact surface.

The term unremoved impurities is used hereinafter and in the appended claims to mean those impurities occurring in the ores from which the germanium is derived and in the reaction agents utilized to derive the high-purity germanium and which remain in the germanium after the purification process has been completed.

Also, a method of producing a crystal rectifier element in accordance with the invention comprises etching a contact surface on a body of germanium of high purity.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description, and its scope will be pointed out in the appended claims.

The effectiveness of the nonlinear or rectifying action of a crystal is to be measured by the value of the back-to-forward resistance ratio; that is to say, the ratio of the higher resistance in one direction of flow of current through a contact of which the crystal forms part to the lower resistance in the opposite direction of flow of current through the contact; the higher the value of this ratio the better is the rectifying action of the crystal.

It will be understood that where hereinafter any quantitative statement is made concerning the value of the back-to-forward resistance ratio, the crystal body per se comprises one element in contact with the point of a tungsten wire 0.2 mm. in diameter having a pointed contact end which is formed by shearing the wire along a cleavage plane inclined to the axis of the wire at an angle of about 45 degrees. The sheared point is electrolytically etched and polished, the pressure between the'point and the crystal is about 10 grams weight, the voltage applied to measure the resistance is 1.5 volts direct, and the average is taken of a number of readings, for example six, with the metal point in contact with a different part of the contact surface of the crystal for each reading. The contact surface of a semiconducting crystal element is the surface of the element which is adapted to abut against the other element of the contact, e. g. against a pointed metallic wire contact.

Referring again to the germanium body of the crystal it may be remarked that the improvement sought in accordance with the invention is to be regarded statistically as applied to the production of a number of crystal contacts regarded as a whole. It is, as far as we are aware, impossible to predict with certainty the effect of the etching treatment on any individual element and some elements may in fact exhibit no improvement or may even be worse after the treatment than before. But with any batch of appreciable size the majority usually are improved.

The chemical etching reagent may be either acid or alkaline; thus we have obtained improvements using individually hydrofluoric acid, nitric acid, sodium hydroxide, or ammonia. But we have found it generally preferable to use a reagent consisting of or comprising hydrofluoric acid, and we have obtained the best results with a mixture in the proportions by volume of 1 part of distilled water, 1 part of 50 per cent. hydrofluoric acid, and 1 to 2 parts of concentrated nitric acid.

The crystalline element, or at least the contact surface thereof, may be simply dipped into the' reagent for a length of time which will depend on the nature of the reagent and possibly also on the sample of germanium being used; the optimum time is best determined by experiment for each particular case. For guidance it may be said that, for the reagents mentioned, vwe have obtained improvements with dipping for the following lengths of time:

40 seconds with 50 per cent. hydrofluoric acid.

30 seconds with concentrated nitric acid.

seconds with a mixture of 1 part of distilled water, 1 part of 50 per cent. hydrofluoric acid, and 1 part of concentrated nitric acid.

60 seconds with per cent. aqueous sodium hydroxide solution.

120 seconds with .88 liquid ammonia.

We have found it generally preferable to use a natural cleavage surface of the germanium as the contact surface to be treated, but the etching treatment in accordance with the invention usually also improves polished contact surfaces.

The following figures are given by way of example to indicate the improvement obtainable by the treatment in accordance with the invention. The etching reagent used was a mixture of, by volume, 1 part of 50 per cent. hydrofluoric acid, 1 part of concentrated nitric acid, and 1 part of distilled water. A natural cleavage surface was treated in each case, and the time of dipping was 10 seconds for each crystal.

In some cases we have obtained improved results by heating the germanium in air to a temperature of about 600-700 degrees centigrade for about half an hour before submitting it to the etching treatment.

As indicated, the germanium used should be of a high degree of purity, and preferably is of a spectroscopic purity of not less than 99.95 per cent. We have, however, obtained satisfactory results with a lower purity of about 99.90 per cent, but germanium whose purity was believed to be as low as 99.24 per cent. gave much poorer results.

We have found that a suitable method for purifying the germanium to the requisite extent is to distill the tetrachloride of the metal from a hydrochloric acid solution in which there is maintained some excess of chlorine or chloride ion. The distilled tetrachloride is received in distilled water where it is hydrolysed and a hydrated germanium dioxide compound is precipitated. The dioxide compound is separated, and then it may be dissolved in hydrochloric acid to reform the tetrachloride and the process repeated several times if necessary until germanium dioxide of a predetermined purity is obtained. The purified germanium dioxide then is reduced in a current of pure dry hydrogen and the resulting germanium powder fused in a vacuum or in an inert atmosphere and cooled to form the solid crystalline metal, which is subsequently broken into fragments and otherwise shaped if necessary to provide individual contact elements.

It may be noted that rectifier elements made with germanium are usually negative; 1. e. such that electrons flow from the crystal to the metal point, assuming a metal point to be used, whereas contacts made with commercial silicon are usually positive, i. e. such that electrons flow from the metal to the crystal. However, crystals of germanium that give positive contacts have been found. It is likely on theoretical grounds that the difference of sign with different germanium crystals is associated with differences in the content of some very minute trace of impurity, possibly oxygen. Such positive contact crystals are of comparatively rare occurrence and it will be understood that they are to be regarded as anomalies.

Finally it may be observed that germanium elements manufactured in accordance with the invention often have one notable advantage over most silicon crystals; with both typesthe aforesaid back-to-forward resistance ratio decreases rapidly with time if the current passed through the contact exceeds some limit, but, whereas with elements of commercial silicon the limit is usually 10-24 ma., with germanium elements in accordance with the invention it is often many times greater. This may be expressed more appropriately by stating that the germanium elements usually can withstand materially greater voltage across the contact in the high resistance direction (i. e., tending to send current in the direction in which the contact has the higher resistance) than can elements made with ordinary commercial silicon.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A crystal rectifier element comprising a body of germanium having unremoved impurities in an amount not exceeding 0.10 per cent, and having an etched crystallographic cleavage contact surface.

2. An electrical translating device comprising a body of germanium of high purity having an etchedcrystallographic cleavage contact surface and a pointed metallic contact held against said contact surface whereby the device may serve as a circuit component presenting a nonlinear impedance to the passage of electric current therethrough.

3. The method of producing a crystal rectifier comprising shaping a body of germanium of high purity to form a'crystallographic cleavage contact surface on said body and subsequently etching said contact surface.

4. The method of producing a crystal rectifier element comprising, forming a contact surface on a body of germanium of high purity and etching said surface with a mixture having the approximate proportions by volume of one part of water, one part to two parts of concentrated nitric caid, and hydrofluoric acid equivalent to that obtained by including in said mixture one part of 50 per cent. hydrofluoric acid.

5. The method of producing a crystal rectifier element comprising, forming a contact surface on a body of germanium of high purity, heating at least said contact surface of said body of germanium in air to a temperature of 600 to 700 degrees centigrade for approximately half an hour, and subsequently etching said surface.

6. The method of producing a crystal rectifier element comprising, reducing a germanium dioxide compound with dry hydrogen gas to form metallic germanium powder, fusing said germanium powder in the absence of chemically active gases, cooling the fused germanium to form an uncomminuted mass of germanium of high purity, and shaping at least a part of said uncomminuted mass into a crystalline body of convenient size having a surface suitable for application of a metallic contact thereto.

7. The method of producing a crystal rectifier element comprising, reducing a germanium dioxide compound with dry hydrogen gas to form metallic germanium powder, fusing said germanium powder in the absence of chemically active gases, cooling the fused germanium to form an uncomminuted mass of germanium of high purity, shaping at least a part of said uncomminuted mass into a crystalline body of convenient size having a contact surface, and etching said contact surface.

8. The method of producing a crystal rectifier element comprising, distilling germanium tetrachloride from hydrochloric acid solution, receiving the distilled tetrachloride into aqueous solution to effect hydrolysis and precipitate a germanium dioxide compound, reducing said dioxide compound with dry hydrogen gas to form metallic germanium powder, fusing said germanium powder in the absence of chemically active gases, cooling the fused germanium to form an uncomminuted mass of germanium of high purity, and shaping at least a part of said uncomminuted mass into a crystalline body of convenient size having a surface suitable for application of a metallic contact thereto.

9. The method of producing a crystal rectifier element comprising, distilling germanium tetrachloride from hydrochloric acid solution, receiving the distilled tetrachloride into aqueous solution to effect hydrolysis and precipitate a germanium dioxide compound, reducing said dioxide compound with dry hydrogen gas to form metallic germanium powder, fusing said germanium powder in the absence of chemically active gases, cooling the fused germanium to form an uncomminuted mass of germanium of high purity, shaping at least a part of said uncomminuted mass into a crystalline body of convenient size having a contact surface, and etching said contact surface.

10. The method of producing a crystal rectifier element comprising: distilling germanium tetrachloride from hydrochloric acid solution; receiving the distilled tetrachloride into aqueous solution to effect hydrolysis and precipitate a germanium dioxide compound; dissolving said precipitated dioxide compound in hyrochloric acid to reform germanium tetrachloride and repeating said operations of distilling and hydrolysis, the cycle of dissolving, distilling, and hydrolysis being carried out until a precipitate of germanium dioxide compound of a predetermined purity is obtained; reducing said last-mentioned dioxide compound with dry hydrogen gas to form metallic germanium powder; fusing said germanium powder in the absence of chemically active gases; cooling the fused germanium to form an uncomminuted. mass of germanium of a predetermined high purity; shaping at least a part of said uncomminuted'mass into a crystalline body of convenient size having a contact surface; and etching said contact surface.

DOUGLAS E. JONES. JOHN W. RYDE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,708,571 Hartmann et a1 Apr. 9, 1929 2,266,922 Thompson et al Dec. 23, 1941 2,402,661 Ohl June 25, 1946 2,402,839 Ohl June 25, 1946 OTHER REFERENCES Merritt, Proc. Natl Acad. of Science, vol. II. 1925. 

2. AN ELECTRICAL TRANSLATING DEVICE COMPRISING A BODY OF GERMANIUM OF HIGH PURITY HAVING AN ETCHED CRYSTALLOGRAPHIC CLEAVAGE CONTACT SURFACE AND A POINTED METALLIC CONTACT HELD AGAINST SAID CONTACT SURFACE WHEREBY THE DEVICE MAY SERVE AS A CIRCUIT COMPONENT PRESENTING A NONLINEAR IMPEDANCE TO THE PASSAGE OF ELECTRIC CURRENT THERETHROUGH.
 3. THE METHOD OF PRODUCING A CRYSTAL RECTIFIER COMPRISING SHAPING A BODY OF GERMANIUM OF HIGH PURITY TO FORM A CRYSTALLOGRAPHIC CLEAVAGE CONTACT SURFACE ON SAID BODY AND SUBSEQUENTLY ETCHING SAID CONTACT SURFACE. 